If you scale up customers, you scale up support. If you don't want to serve new customers, tell them to take their business elsewhere. But if you want to be a big boy, you need to play like you are one.
If your maximum addressable market is “the whole economy,” as seen in SpaceX filings, then a city-sized call centre (distributed, of course) really is ‘t that much of an ask.
If you think you hit a limit where you can't support any new customer anymore, you just tell them you have no capacity at the moment. Like any normal practice does.
